SELECT with expressions and calculations in SQL - Time & Space Complexity

Time Complexity: SELECT with expressions and calculations

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a SELECT query with calculations changes as the data grows.

How does adding more rows affect the work the database does?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT product_id, price, quantity, price * quantity AS total_cost
FROM sales;

This query selects each sale's product ID, price, quantity, and calculates the total cost by multiplying price and quantity for each row.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The database reads each row in the sales table once.
• How many times: Once per row, it performs the multiplication calculation.

How Execution Grows With Input

As the number of rows grows, the database does more calculations, one for each row.

Input Size (n)	Approx. Operations
10	10 multiplications
100	100 multiplications
1000	1000 multiplications

Pattern observation: The work grows directly with the number of rows; double the rows, double the calculations.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows in a straight line with the number of rows in the table.

Common Mistake

[X] Wrong: "Calculations inside SELECT make the query much slower than just selecting columns."

[OK] Correct: The calculation is simple and done once per row, so it adds only a small, direct cost proportional to the number of rows.

Interview Connect

Understanding how simple calculations affect query time helps you explain performance clearly and shows you know how databases handle data row by row.

Self-Check

"What if we added a JOIN to another table with m rows? How would the time complexity change?"

Practice

1. What does the SQL expression SELECT price * quantity AS total_cost FROM sales; do?

easy

A. Calculates total cost by multiplying price and quantity for each row.

B. Adds price and quantity values together for each row.

C. Divides price by quantity for each row.

D. Filters rows where price equals quantity.

Solution

1. Step 1: Understand the multiplication in SELECT

   The expression multiplies the values in columns price and quantity for each row.

2. Step 2: Recognize the alias usage

   The result of multiplication is renamed as total_cost for clarity in output.

3. Final Answer:

   Calculates total cost by multiplying price and quantity for each row. -> Option A

4. Quick Check:

   price * quantity = total_cost [OK]

Hint: Look for * operator to identify multiplication [OK]

Common Mistakes:

• Confusing multiplication with addition or division
• Ignoring the AS alias
• Thinking it filters rows

2. Which of the following is the correct syntax to calculate the average price and rename it as avg_price?

easy

A. SELECT AVG(price) AS avg_price FROM products;

B. SELECT AVG(price) avg_price FROM products;

C. SELECT AVG(price) TO avg_price FROM products;

D. SELECT AVG(price) RENAME avg_price FROM products;

Solution

1. Step 1: Identify correct alias syntax

   In SQL, the keyword AS is used to rename columns or expressions in SELECT.

2. Step 2: Check each option's syntax

   SELECT AVG(price) AS avg_price FROM products; uses AVG(price) AS avg_price which is the correct syntax; others use invalid keywords or omit AS.

3. Final Answer:

   SELECT AVG(price) AS avg_price FROM products; -> Option A

4. Quick Check:

   Use AS to rename columns [OK]

Hint: Use AS keyword to rename calculated columns [OK]

Common Mistakes:

• Omitting AS keyword
• Using TO or RENAME instead of AS
• Misplacing parentheses

3. Given a table orders with columns price and quantity, what is the output of this query?

SELECT price, quantity, price * quantity AS total FROM orders WHERE price = 10;

medium

A. All rows with total calculated, ignoring price filter.

B. Syntax error due to missing GROUP BY.

C. Rows where price is 10, showing price, quantity, and total (price*quantity).

D. Rows where quantity equals 10.

Solution

1. Step 1: Understand the WHERE clause effect

   The query filters rows to only those where price equals 10.

2. Step 2: Calculate total for filtered rows

   For each filtered row, it shows price, quantity, and total which is price multiplied by quantity.

3. Final Answer:

   Rows where price is 10, showing price, quantity, and total (price*quantity). -> Option C

4. Quick Check:

   WHERE price=10 filters rows; total = price*quantity [OK]

Hint: Check WHERE filters before calculations [OK]

Common Mistakes:

• Ignoring WHERE clause filtering
• Expecting all rows instead of filtered
• Thinking GROUP BY is needed here

4. Identify the error in this SQL query:

SELECT price + quantity AS total FROM sales WHERE total > 100;

medium

A. Missing FROM clause in the query.

B. You cannot use alias 'total' in WHERE clause directly.

C. The + operator is invalid for numeric addition.

D. AS keyword cannot be used for aliasing.

Solution

1. Step 1: Understand alias scope in SQL

   Aliases defined in SELECT cannot be used in WHERE clause because WHERE is evaluated before SELECT.

2. Step 2: Identify correct fix

   Use the full expression (price + quantity) in WHERE or use a subquery/CTE to filter by total.

3. Final Answer:

   You cannot use alias 'total' in WHERE clause directly. -> Option B

4. Quick Check:

   Alias not usable in WHERE [OK]

Hint: Aliases appear after WHERE; can't use them there [OK]

Common Mistakes:

• Using alias in WHERE clause
• Thinking + is invalid operator
• Ignoring query evaluation order

5. You want to calculate the discounted price for products with a 10% discount and show it as discounted_price. Which query correctly applies the discount and rounds the result to 2 decimals?

hard

A. SELECT price - (price * 0.10) AS discounted_price FROM products;

B. SELECT price * 0.90 AS discounted_price FROM products;

C. SELECT ROUND(price * 0.90) AS discounted_price FROM products;

D. SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products;

Solution

1. Step 1: Calculate 10% discount correctly

   Discounted price is price minus 10% of price, which is price - (price * 0.10) or price * 0.90.

2. Step 2: Round the result to 2 decimals

   Use ROUND(expression, 2) to round to two decimal places for currency formatting.

3. Final Answer:

   SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products; -> Option D

4. Quick Check:

   Use ROUND(..., 2) for 2 decimals [OK]

Hint: Use ROUND(expression, 2) to round decimals [OK]

Common Mistakes:

• Not rounding the result
• Rounding without specifying decimals
• Using incorrect discount calculation